1. Field of the Invention
The present invention relates generally to the field of electrophotographic copiers, and, more particularly, to an improved toner dispenser for use in the developer assembly of an electrophotographic copier.
2. Description of the Prior Art
In electrophotographic copiers utilizing typical cascade development techniques, a latent electrostatic image is formed on a photoconductive insulating member and then developed by cascading a developer mixture over the image as it is carried through a development zone. This developer mixture typically consists of relatively large particles or carrier beads and many smaller pigmented toner particles which electrostatically cling to the carrier, and as this developer mixture cascades through the development zone and over the latent electrostatic image, toner particles are separated from the carrier and deposited on the image to develop it. After completing its passage through the development zone, the developer mixture is then generally accumulated in a sump in the bottom of the development unit to be eventually picked up by a suitable conveyor and returned to the top of the unit to be dropped back through the development zone to develop later images.
Because, during each development, toner is pulled off of the carrier beads and deposited on the electrostatic image, it is necessary that this toner be replaced to ensure that the developer mixture will always have an adequate concentration of toner. Accordingly, most copying machines additionally include some form of toner dispenser to supply fresh toner to the developer prior to its being returned to the top of the development zone.
Although a large variety of toner dispensers have been developed and are being used in the prior art, most of them are not fully satisfactory for a variety of reasons. For one thing, many of the prior art toner dispensers are sensitive to external factors such as the amount of toner in the supply hopper, machine vibrations and the like. Accordingly, the rate at which toner is dispensed is frequently not controllable to the extent and to the accuracy desired. Also, many of the existing systems are relatively complicated and require frequent repair. Furthermore, because of their physical locations, which are dictated by developer assembly space and toner storage capacity limitations, most of the toner dispensers presently being used are not designed to assist in mixing the fresh toner with the developer. In this regard, it is desirable that the developer and the fresh toner be well mixed prior to its being re-cascaded through the development zone because it is this mixing activity which triboelectrically charges the toner so that it can be properly deposited on the latent electrostatic image. With many of the systems available today, it has been found necessary to use rotating screws or some other type of mechanical mixer to ensure proper mixing of the fresh toner with the developer and this mechanical mixing is not desirable because it adds complexity and cost and can excessively abrade the developer.
In general, there is a need for an improved toner dispensing apparatus which can reliably dispense metered amounts of fresh toner to the developer and which is designed in such a manner that ample mixing of the fresh toner with the developer can be accomplished in a simple and efficient manner.